Manhès–David process

The Manhès–David process is a refining process of the copper mattes, invented in 1880 by the French industrialist Pierre Manhès and his engineer Paul David. Inspired by the Bessemer process, it consists of the use of a converter to oxidise with air the undesirable chemical elements (mainly iron and sulfur) contained in the matte, to transform it into copper. The quantity of the elements to be oxidized, as well as the low heat produced by the chemical reactions, lead to drastics modifications of the converter. Manhès and David designed it as a horizontal cylinder, with nozzles aligned from one end to the other. A few years later, the Americans engineers William H. Peirce and Elias Anton Cappelen Smith lined it with basic refractory materials, much more durable than that used by the French inventors. While this improvement does not alter the principles of the process, it eases its widespread use, accelerating the switchover of copper production from Britain to the United States. At the beginning of the 21st century, the Pierce-Smith converters refine 90% of the copper mattes and is used in 60% of the nickel extracted. This converter, like the addition of pure oxygen, the automation of the running, the treatment of smoke and the increasing size of the tools, ensured the durability of the Manhès–David process, even if modern tools have little relationship with their ancestors.

Origins of the process

Relationship with the Bessemer process

Just as iron produced by a blast furnace comes out alloyed with other chemical elements as cast iron, copper extracted from ore becomes an alloy with sulfur, iron, etc. called matte. To apply the same purification processes to these two metals is therefore logical. Applying the Bessemer process to copper metallurgy was proposed, and the principle validated in 1866, ten years after Henry Bessemer's invention, by the Russian engineer Semenikow. The refinement of the alloy in the converter is possible because the combustion of undesirable elements is strongly exothermic: the oxidation of silicon and carbon respectively produce 32.8 and 10.3 kilojoules per kilogram. On the other hand, if a copper matte contains an abundance of iron and sulfur, these elements must first be separated (which consumes 6.8 kilojoules per kilogram of FeS) before their oxidation (which only produces 5.9 and 9.1 kJ/kg respectively) can begin.

First attempts The first refinements of copper alloys by a converter took place in Ducktown, Tennessee where A. Raht worked on a partial refinement of the matte from 1866 to 1875. In 1867, the Russians Jossa and Latelin tried to experimentally verify the studies of Semenikow. In 1870, they stopped their experiments after only having succeeded to increase the copper content from 31% to 72-80%. In England, John Hollway continued these trials until 1878. Like his predecessors, he observed that if blowing began in a satisfactory manner, it became more and more intermittent as the refinement progressed. The obstacles he encountered were numerous:

The weight of produced slag was equal to that of copper and its volume was much greater than that in the converter. It was thus necessary to drain the retort regularly. The density of the molten metal changed greatly (with copper having a density three times as great as the pyrite from which it is made). The duration of the air blowing, which can reach two hours, involved large thermal losses. The siliceous refractory material was absorbed by the slag, in which it acted as flux. All of the encountered difficulties could not be easily resolved: the thermal heat balance of the refinement reaction in air of copper was not as favorable as for iron, and the matte solidified in the tuyeres before being refined. Even when modified, a Bessemer converter was capable at best of removing iron and a portion of sulfur. Hollway failed, but by publishing all of the details of his experiments, he identified the essential problems.

Lateral tuyeres In the 1870s, the French industrialist Pierre Manhès began his first attempts with a small, ordinary Bessemer converter of 50 kg in his factory in Vedène, then in factories in Éguilles, near Avignon. He sought to refine a matte with 25 to 30% copper previously melted in a crucible. But like Hollway, he did not succeed in completely refining the matte. The oxidation of undesirable elements occurred as expected, but the operation was quickly disrupted by the appearance of metallic copper. The matte, which was an ionic compound, was immiscible with the slag, but also with the molten metal. The latter, which is denser (ρcopper ≈ 9), went to the bottom of the converter and clogged the tuyeres. Pierre Manhès then patented the use of additives whose oxidation would release enough heat to avoid getting stuck. In the end, it was the Frenchman Paul David, then an engineer in his factory in 1880, who suggested the solution. He proposed horizontal tuyeres placed at a sufficient distance from the bottom of the converter so that the copper could gather below them and the air blow constantly in the matte. By 1881, their converter was both technically operational and cost-effective. In the autumn of 1884, the process was adopted in the United States by the Parrot Silver and Copper Company in Butte, Montana. The two types became larger and larger, increasing from a capacity of one ton to eight tons in 1912, and even fifteen tons for cylindrical converters in 1920.

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